Electrochemistry and Half Reactions

enigmatic

The question:
Balance each of the following skeletal equations by using oxidation and reduction half reactions. All the reactions take place in acidic solution. Identify the oxidizing agent and reducing agent in each reaction.
a) Cl₂ (g) + S₂O₃^2-(aq) --> Cl^-(aq)+SO₄^2-(aq)

My answer so far:
Half Reaction for chlorine: Cl₂ + 2e^- --> Cl^-

This matched the back of the book

The problem:
The half reaction I wrote for S₂O₃ was completely different from what was in the back of the book.

The answer from the back of the book:
S₂O₃^2-(aq) + 5H₂O(l) --> 2SO₄^2-(aq) + 10H⁺(aq) + 8 e^-

Would someone please walk me through how to come to this answer? I would very much appreciate it!

Thanks! ;D

Gokul43201

That should really be 2Cl^- on the right, but you got the basic idea right.

Okay, this step is clearly a little more tricky than the first one.

You start with:

1. S₂O₃^2-(aq) + unknowns --> SO₄^2-(aq) + unknowns

The first thing to recognize is that you want to fix the number of S atoms on the right (don't worry about O atoms as there's always H2O that can be used to fix that). 2 S atoms on the left, so you want 2 S atoms on the right.

2. S₂O₃^2-(aq) + unknowns --> 2SO₄^2-(aq) + unknowns

Next, you fix the number of O atoms by adding H2O to the required side. In this case, we've got 3 O atoms on the left and 8 on the right. So we need 5 more on the left, and the way to do that is by adding 5 H2O to the left.

3. S₂O₃^2-(aq) + 5H₂O(l) + unknowns --> 2SO₄^2-(aq) + unknowns

Now you see that you've got 10 H atoms on the left and none on the right. You fix this by adding 10 H+ ions (you're allowed to do this, if the reaction takes place in acidic medium) to the right.

4. S₂O₃^2-(aq) + 5H₂O(l) + unknowns --> 2SO₄^2-(aq) + 10H⁺(aq) + unknowns

Now all elements are balanced, but the charge is not. On the left, you have -2. On the right you have -4 + 10 = +6. So you need to add 8 electrons on the right to balance the charge.

5. S₂O₃^2-(aq) + 5H₂O(l) --> 2SO₄^2-(aq) + 10H⁺(aq) + 8 e^-

Now you've got both charge and chemical elements balanced, and you're done.

enigmatic

Thanks so much!

So you balance everything except hydrogen and oxygen,
balance oxygen by adding water to the other side,
balance hydrogen by adding hydrogen ions to the other side,
and balance charge by adding electrons on the side that needs it?

Would you say that those would be the proper steps?

Borek

Balancing redox reaction with half-cell method.

Your approach will work for acidic solutions. If you know solution is basic and you need oxygen add two OH^- where you need oxygen and H₂O on the second side of the equation.

enigmatic

thanks! (and thanks for the link, too!)

chemisttree

I don't think it is balanced yet...